Internal combustion engines operate by the controlled combustion of hydrocarbon fuels and produce exhaust gases containing complete combustion products such as carbon dioxide (CO2) and water (H2O), and incomplete combustion products such as carbon monoxide (CO) and unburnt hydrocarbons (HC). Further, due to the very high temperatures produced by the burning of the hydrocarbon fuels, thermal fixation of nitrogen in the air results in the detrimental formation of nitrogen oxide compounds (NOx). Certain compounds in the exhaust are undesirable in that they must be controlled in order to meet government emissions regulations. Among the regulated compounds are hydrocarbons, soot particulates, and NOx.
The quantity of pollutants generated by incomplete combustion varies with operating conditions of the engine, but is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. Conditions conducive to reducing carbon monoxide and unburnt hydrocarbons (a fuel mixture just lean of stoichiometric and high combustion temperatures) cause an increased formation of NOx and conditions conducive to reducing the formation of NOx (rich fuel mixture and low combustion temperatures) cause an increase in carbon monoxide and unburnt hydrocarbons in the exhaust gases. As a result, within the region of stable operation of the internal combustion engine, a significant amount of CO, HC and NOx is emitted from the engine.
Current trends in the automotive industry are driven primarily by more stringent pollution and fuel economy regulations. Diesel and lean burn engines are attractive for a next generation vehicle in view of the fuel economy. However, a NOx-controllable stoichiometric design has not been developed for diesel and lean burn engines.
Excessive oxygen in lean-burn engine exhausts can inhibit NOx removal in conventional three-way catalytic converters. The exhaust stream from a diesel engine has a substantial oxygen content, from perhaps about 2–18% oxygen, and, in addition, contains a significant amount of particulate emissions. The particulate emissions, or soot, are thought to be primarily carbonaceous particles. The excess oxygen makes lean NOx catalytic processing inefficient and fraught with limitations such as temperature windows and sulfur poisoning. Therefore, an effective and durable catalyst for controlling NOx emissions under net oxidizing conditions is critical for diesel engines.
NOx adsorbers have shown some promise but durability concerns, sulfur poisoning and rich purging requirements have limited their commerciality. NOx catalysts having activity, durability, and the temperature window to effectively remove NOx from the exhaust have not been successfully developed. Conventional lean NOx catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges. Conventional catalysts are therefore inadequate for lean-burn operation and ordinary driving conditions. An alternative is to use catalysts that selectively reduce NOx in the presence of a co-reductant, e.g., selective catalytic reduction (SCR) using ammonia as a co-reductant. A more active approach such as urea injection with SCR has demonstrated good NOx control, but the added complexity of the urea injection and the lack of a distribution infrastructure are significant detractors.
As an alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust, a non-thermal plasma system has been introduced. The unique requirements of a vehicular non-thermal plasma system include high efficiency in not only NOx reduction but in power generation and control; minimal size, weight, and cost; and durability greater than 100,000 miles. However, these systems suffer from serious shortcomings. First, such systems are run continually, which results in a relatively large power consumption per unit of material destroyed, particularly when used to treat low concentration of emissions in exhaust streams. A more concentrated emission stream would allow less power consumption per molecule of pollutant destroyed. Secondly, when the non-thermal plasma reactor is operated under oxidizing conditions, nitrous oxide tends to be converted into undesirable nitric oxide and nitric acid. The nitric oxide and nitric acid must then be collected and separately treated or disposed of. In the case of automotive exhaust, the undesirable products cannot be easily collected and disposed of. It would be desirable, therefore, to only operate the non-thermal plasma reactor with a non-oxidizing atmosphere. However, as mentioned, it is desirable for reasons of fuel economy to operate an automotive engine under lean burn conditions for as much of the time of operation as possible.